When such vehicles are running on roads or else on runways, usually consisting of a pavement of the asphalt or concrete type, the grip properties of the tires with which said vehicle is fitted are fundamental as regards the transmission of accelerating and braking torques and as regards the stability of the vehicle. The term “stability” of the vehicle is understood in particular to mean the guiding or maintaining of the path of the vehicle.
The grip of tires on the pavement which, for example consists of a layer of asphalt, essentially is the result of a phenomenon whereby the rubber compound of the tire tread undergoes indentation by said tread sliding on the asperities of said asphalt layer, which asperities are forced into the surface of said rubber compound. This sliding-induced indentation phenomenon, associated with the hysteretic properties of the rubber compound, which deforms, creates forces, opposed to the sliding, between the tire and the pavement, which result in the tire gripping said pavement.
It is clearly apparent in view of this grip phenomenon that the appearance of the surface of the pavement on which said vehicle runs will play a major role in determining the amount of grip. It is also known to those skilled in the art that the roughness of the pavement, or the size of the asperities of said pavement, has an effect on the amount of grip through the size of the aggregates constituting said pavement and also through the surface finish of said aggregates. The term “macroroughness” is used in relation to the size of the aggregates corresponding to dimensions of the order of one millimeter and the term “microroughness” is used as regards their surface finish, corresponding to dimensions of the order of one micron. The term “medio-roughness” (or intermediate roughness) is also used as being the appearance of the aggregates on a scale intermediate between the two dimensional scales, i.e. of the order of ten microns or so.
It is therefore usual to seek to identify the nature of the pavements on which the vehicles have to run in order to take these parameters into account when designing tires. This type of identification becomes more important when it is necessary to analyze the behavior of a tire rolling on a given pavement.
Thus, methods are known for estimating the profile of a pavement on which vehicles fitted with tires run. It is thus usual to seek to define either the profile in terms of macroroughness, especially for the purpose of establishing the capability of the pavement to remove water in wet weather, or the profile both in terms of macroroughness and microroughness when it is intended to identify the grip of a tire on said pavement.
In particular, on-road measurement techniques are known which consist, for example, in taking an impression using a carbon paper or else simple paper associated with a deposit of ink on the pavement. Such measurements allow the contact area of a tire on a given pavement to be determined from a macroscopic viewpoint. However, such measurements are particularly imprecise and poorly reproducible.
Other known measurement techniques consist in taking a sample of the pavement, for example by a coring technique, so as subsequently to carry out measurements in a laboratory. The measurements carried out may therefore be very precise, including measurements on a scale on the order of one micron. Such methods have the drawback of being destructive, since they require samples to be taken. Since such analysis of the surface of a pavement is advantageously carried out at several randomly selected points so as to obtain a statistical representation of said pavement, it is clear that these destructive methods are of little interest owing to the damage that they cause.
The measurement techniques intermediate between those already described consist in taking an imprint by molding a pavement and then analyzing said molding or a counter-molding in the laboratory. The molding may for example be carried out using a material of the dental paste type. It turns out that the materials used limit the analysis of the pavement profile to macroscopic dimensions, the microscopic profile of said pavement disappearing during molding.
Also known, in particular from documents US 2003/0000097, U.S. Pat. No. 5,790,243 and SE 527 952, are two-dimensional measurement methods that consist of many measurements so as to determine a transverse macroscopic profile of a road, in particular the rutting formed by vehicle traffic. The transposition of these techniques to smaller scales, and especially that of the order of one micron as explained above, would in particular require time-dependent deviations such as thermal expansion effects to be overcome. This is because applying such methods with a resolution of the order of one micron would require a number of measurements such that the time needed for the measurements would be very long and would result in a loss of precision owing to environmental physical effects which, over such periods of time, would change sufficiently to create measurement variations that are unacceptable with the required precision.